1. Field of the Invention
The present invention relates to an organic light emitting device, and more particularly, to a top-emitting organic light emitting device having an improved pixel electrode layout for decreasing photo-leakage of a thin film transistor and enhancing aperture ratio.
2. Description of the Related Technology
In general, an organic light emitting device employs an organic light emitting diode made of a fluorescent or phosphoric organic compound that can be electrically excited. The organic light emitting diode has a layered structure including an anode, an organic emitting layer and a cathode, and illuminates according to an applied voltage or current. The organic emitting layer has a multi-layered structure including a hole injecting layer and an electron injecting layer, which are formed on either side of an emitting layer to enhance electron and hole injection characteristics. Furthermore, an organic thin film layer can selectively include an electron transporting layer, a hole transporting layer, and a hole blocking layer, to enhance emission characteristics of the organic emitting device.
The organic light emitting device may be a passive matrix type or an active matrix type according to the driving method. For example, a passive matrix device receives a current only when the scan line to which it is connected is selected. The device then uses the current to control the brightness of the pixel.
On the other hand, an active matrix pixel uses a capacitor to store a voltage for controlling the light emission of the pixel. The device applies the stored voltage to the pixel in a period corresponding to a frame. An active matrix display may be a voltage programming type or a current programming type according to the signal applied for storing the voltage in the capacitor.
Further, the organic light emitting device is classified into a bottom-emitting type and a top-emitting type according to a position of a reflecting layer. The bottom-emitting device reflects light emitted from the organic emitting layer in the direction of the bottom of the substrate, and the top-emitting device reflects the light emitted from the organic emitting layer in the direction of the top of the substrate.
FIG. 1 is a layout view of red, green, and blue pixels of a pixel array in a conventional top-emitting organic light emitting device.
Referring to FIG. 1, red (R), green (G) and blue (B) pixels are arranged on a substrate. Connected to each pixel, there are scan lines Sn−1, Sn and Em; a data line DR, DG or DB; and a power supply line Vdd, each line extending in one of first and second orthogonal directions. Further, red, green, and blue pixel driving circuits are provided within the red, green, and blue pixels, respectively. Each pixel driving circuit includes five transistors M1 through M5, and two capacitors Cvth and Cst.
The red, green, and blue pixel driving circuits are connected to red, green, and blue pixel electrodes 15R, 15G and 15B, respectively. In more detail, one of the source and drain electrodes of a thin film transistor M4 included in the green pixel driving circuit is connected to the green pixel electrode 15G through a via hole 14. The via hole 14 is formed on the green pixel electrode 15G, and consequently, this area cannot be used to emit light. Because of the necessity of this Non Light emitting Area NLA, (refer to FIG. 1), the size of the subsequently formed aperture is reduced from optimal and thus the aperture ratio is limited.
Furthermore, in the conventional top-emitting organic light emitting device, each pixel electrode 15R, 15G, and 15B is formed to have a minimum size depending on a design rule. Also, each pixel electrode 15R, 15G, and 15B is designed not to overlap each data line DR, DG, and DB, thereby minimizing crosstalk due to parasitic capacitance. For example, in the case of a 2.2 inch quarter video graphic array (QVGA) having 240×320 pixels, each pixel has a width of 47 μm, and a space TA between neighboring pixel electrodes is 17 μm, so that each pixel electrode can be designed to a width of 30 μm (47 μm−17 μm). As described above, in the conventional top-emitting organic light emitting device, each pixel electrode is designed to a minimum width, and neighboring pixel electrodes are designed to be widely spaced apart from each other by as much as 17 μm. Therefore, the wide space TA between the pixel electrodes leaks emitted light, thereby deteriorating the voltage-current characteristics of the thin film transistors (e.g., M3 and M5 in FIG. 1), i.e., increasing photo-leakage.